1. Field of the Invention
The present invention relates to a servo control device for driving a servomotor and particularly relates to a servo control device having an automatic filter adjustment function based on experimental modal analysis.
2. Description of the Related Art
Many servo control devices use a technique of applying a band-rejection filter to a torque command value, in order to stabilize the servo control systems of machine tools. In filter adjustment, it is common to carry out sine wave sweep or square wave sweep for a speed control loop including a torque command generating unit and a speed detecting unit, to thereby measure a frequency response of the control loop and to obtain a resonance frequency of the control loop.
In consideration that the calculation load on each filter is extremely heavy, it is difficult to increase the number of filters for period calculation in real-time. For this reason, the number of filters is limited by the constraint of the performance of hardware to be used for servo control. Hence, by using a finite number of filters, adjusting the filters to stabilize the control system in an efficient manner is of great importance. For this purpose, there is a need for a technique that enables selective application of a filter that is considered to provide a high stabilization effect, by evaluating an oscillation risk at each resonance frequency.
In view of the fact that automatic filter adjustment is possible in principle as long as resonance is detected, a number of techniques for automatic filter adjustment have been proposed (e.g. Japanese Laid-open Patent Publication No. H06-78575, to be referred to as “Patent Document 1” below). Patent Document 1 discloses automatic adjustment of a notch filter by obtaining amplitude ratios and frequencies from measurement data to carry out parameter adjustment including discretization calculation. However, it is difficult to adjust the width and the depth of the filter with this technique.
While automatic filter adjustment has the advantage that the results of the adjustment vary little, exact rules for the automatic adjustment need to be defined. There have been proposed techniques for adjusting a whole set of servo control parameters including control gain and feedforward while estimating a transfer function (e.g. Japanese Laid-open Patent Publication No. 2008-259271, to be referred to as “Patent Document 2” below). This known technique is advantageous in that the whole adjustment can be made by taking into account stable operation of a control system. Patent Document 2 discloses automatic adjustment of control gain, notch filter, and feedforward. In this technique, the moment of inertia of a mechanical system is identified, the transfer function of the mechanical system is generated, and the inverse transfer function of the generated transfer function (the reciprocal of the transfer function of the mechanical system) is obtained, and thereby a feedforward coefficient is determined. Filter adjustment itself is carried out on the basis of an oscillation frequency and a control gain. However, Patent Document 2 does not disclose any method for clearly defining the width or depth of a filter.
Examples of applying an experimental modal analysis for servo adjustment have been reported (e.g. Japanese Laid-open Patent Publication No. 2006-227793, to be referred to as “Patent Document 3” below). Patent Document 3 proposes a method for determining a group of constants for a transfer function and describes that a gain, a low-pass filter, and a notch filter for servo control can be adjusted by the use of the method. However, Patent Document 3 does not disclose any technique for selecting a resonance frequency.